This invention relates generally to marine seismic cables and more particularly to a mounting for placing seismic transducers at discrete locations along the seismic cable.
In marine seismic exploration, the seismic cable is towed in a submerged fashion behind a vessel, and seismic detectors in the cable receive reflections of acoustic wave energy to map the geologic strata beneath the surface of the earth. The acoustic wave energy is produced by devices well known to those skilled in the art and produces the reflections received at the transducer locations. Transducers are electrically connected to various recording apparatus onboard the vessel, wherein recording and/or processing of the collected seismic data may take place.
In the past, a number of seismic cables have been available for use in this type of seismic exploration. For instance, one streamer cable comprises a longitudinal tubular member which contains a plurality of spaced hydrophone transducers, with a number of wires interconnecting the hydrophones disposed within the tubular member. The tubular member may be filled with a liquid, such as oil or kerosene, to provide proper buoyancy.
However, one drawback with the use of such liquid-filled cables has been their susceptibility to damage. This occurs because the tubular member must necessarily have thin walls to allow for the transmission of acoustic wave energy. If this thin wall should rupture, kerosene leaks, the buoyance of the cable is lost and the hydrophones contained within the tubular member may be damaged or destroyed. In addition to this disadvantage, the transducers mounted within the hollow tubular member must be insulated from the effects of the oil or kerosene filling the cable and, of course, from any water which may find its way into the cable due to leakage. The hydrophones are easily susceptible to damage or breakage during deployment or handling of the streamer cable.
Another drawback with such liquid filled cables is their large size and weight. In order to maintain neutral buoyancy, a large volume of liquid is required since the buoyance is controlled by the difference in specific gravity between the liquid inside the cable and the water in which the cable is towed. Foam as herein described has a much greater buoyancy and therefore results in a smaller, lighter cable.
Another cable which has solved some of the above problems has been a seismic cable of solid construction which utilizes a solid to provide buoyancy as opposed to a liquid. Typically, a stress member is provided to allow towing of the solid seismic streamer cable. This stress member may be a single steel cable, or may be a synthetic with sufficient tensile strength to support the seismic cable. The electrical conductors needed to interconnect the hydrophones or transducers with each other and with the shipboard electronics may be located within the stress member or may surround the stress member in some fashion. Various types of hydrophones have been utilized with the solid cable construction, two of which will now be described.
One type of hydrophone which has been used with the solid cable construction is a piezoelectric ceramic of cylindrical construction which is mounted concentrically around the central core of the streamer cable, such as that described in U.S. Pat. No. 3,739,326, issued to Kerr et al. An air gap is provided beneath the cylinder to allow the piezoelectric element to respond to impinging seismic energy. This type of hydrophone suffers loss of sensivitity when water leaks into the air gap, which may occur when the cable is submerged and subsequently damaged. Another type of hydrophone for use in a cable of solid construction is described in U.S. Pat. No. 3,781,778 issued to Sawin et al. A disc-type hydrophone is mounted within a rigid case, which case is mounted on an eccentrically placed cable core of the seismic streamer cable. A gel is used to couple an outer covering of the case to the hydrophone so that acoustic wave energy is received by the hydrophone and transmitted to the shipboard electronics. The case construction is such that the entire structure conforms to the profile of the seismic cable. A polyurethane jacket is extruded over the case after it has been placed in a cavity of the hydrophone cable to provide a relatively smooth outer surface. However, this type of hydrophone mounting has been found susceptible to loss of response from puncture of the cable adjacent the rigid mountings and the resultant entry of sea water, or more frequently, creation of vapor pockets in the cavity due to diffusion. In addition, hydrophone repair is made difficult because of the jacket which has been extruded over the hydrophones.
Both of the above hydrophone mounts are fairly sensitive to cable-borne noise. Cable stress member perturbations such as longitudinal, lateral, or torsional waves which are picked up by hydrophones are referred to as cable-borne noise. For instance, sealed cavities are very sensitive to mount deformations induced by stress member perturbations. The deformation is increased where the stress member is mounted to one side of the cable. The nonsymmetrical construction produces larger and nonuniform deformations than a symmetrical cable.